Computer assisted tomography, commonly called CAT or CT scanning, is a method of producing a density image of a cross section of a scanned object, usually a cross section of a human patient. In a CAT scanner of the third generation type, an x-ray source and an array of x-ray detectors are rotated in a scanning plane about a central area in which is located the patient object. X-rays from the source pass from a source point, commonly referred to as the focal spot, through the object and are received by individual detectors. The detectors are arranged in a single row in the shape of an arc of a circle having a center of curvature coincident with the focal spot of the x-ray source. Each detector, therefore, subtends an equal angle relative to the focal spot so that all of the detectors are each theoretically subjected to the sane x-ray exposure levels from an tinattenuated beam during an intensity measurement.
The x-rays that are detected by a single detector at a measuring instant during a scan is considered a "ray." The ray is partially attenuated by all the mass in its path so as to generate a single intensity measurement as a function of the attenuation, and thus the density of the mass in that path. A projection or view, i.e., the x-ray intensity measurement, is typically done at each of a plurality of angular positions for a given position of the disk.
An image reconstructed from data acquired at all of the projection angles during the scan will be a cross-sectional slice in the scanning plane through the object being scanned. In order to "reconstruct" a density image of the cross-section of the object, the image is typically "back projected" using a mathematical algorithm attributed to Radon. Back projection usually involves the reconstruction of the image in a pixel array, wherein each pixel in the array is attributed to a value representative of the density of that volume of the patient which has the area of the pixel and the height equal to the slice thickness. As the source and detectors rotate around the object, rays penetrate the object from different directions, or projection angles, passing through different combinations of pixel locations. The density distribution of the object in the slice plane is mathematically generated from these measurements, and the brightness value of each pixel is set to represent that distribution. The result is an array of pixels of differing values which represents a density image of the slice plane.
Because of the operation of the Radon algorithm, however, inconsistencies in the exposure measurements by any detectors result in artifacts such as extraneous rings appearing in the image. One cause of such inconsistencies is relative motion between source, object and detectors. Contributions to this cause are addressed, for example, in co-pending applications U.S. Ser. No. 08/190,945, filed Feb. 3,1994, in the names of John Dobbs and David Banks and entitled "Modular Detector Arrangement for X-ray Tomographic Detector System" (Attorney's Docket No. ANA-23); and U.S. Ser. No. 08/191,428, filed Feb. 3, 1994, in the names of Bernard M. Gordon, John Dobbs and David Banks and entitled "X-ray Tomography System for and Method of Improving the Quality of a Scanned Image" (Attorney's Docket No. ANA-44), both being assigned to the present assignee and both being filed simultaneously with this application.
Another cause of inconsistency in the exposure measurements by any of the detectors that tends to obscure image detail is an uncompensated change in detector sensitivity of one or more detectors relative to the other detectors. The combination of a rapid scan to minimize patient object motion, and the need to differentiate among soft tissues results in the use of and the need to detect very low levels of x-radiation. The most sensitive and therefore potentially useful detectors for this application are solidstate devices made up of a cadmium tungstate scintillating crystal for converting the x-ray energy to light and a semiconductor photo-diode to convert the light to an electrical signal that can be computer processed. Unfortunately, however, both of these devices are very temperature sensitive, and operation of the x-ray source and power supplies generates a large amount of heat that causes the ambient temperature to rise significantly. Furthermore, while the photo-diodes tend to consistently have a similar temperature coefficient, the cadmium tungstate crystals do not. In fact, even the sign of the coefficient may vary from crystal to crystal. A miscalibration of as little as 0.1%, however, can result in visible rings in the reconstructed image. Accurate temperature control or compensation for each of the solid state detectors is therefore an important requirement.
Adequate temperature control of all of the detectors, however, is far from easy. The ultimate solution would seem to be to measure the temperature of each detector and control it with a servo loop. However, with several hundred detectors mounted closely together in an array, and each detector being less than two millimeters in width, such an approach is impractical. Yet, even simple temperature control of the detector array as a single entity presents many problems. Mounting an air conditioning system on the rotating gantry takes up a great deal of scarce space and hinders accessibility to other components. U.S. Pat. No. 4,969,167 that issued to Zupancic et. al. on Nov. 6, 1990, describes a system that supplies ambient air at elevated pressure in the vicinity of the detectors to cool them. While this may indeed reduce the range of inconsistencies among detectors, it does not allow accurate calibration to within 0.1%.
The current invention is driven by the recognition that any practical type of temperature control of the detector array, in which heat is added to or subtracted from the array, will cause thermal gradients within the array. In order to accurately calibrate a detector array with thermal gradients, however, one must duplicate not only the heat sources to which the array is exposed during a scanning operation, but the time delays as well. But in typical operation, CAT scanners experience very non-uniform time schedules based on individual patient needs, duplicate scans required because of inadvertent patient motion, and non-uniform work load. Conversely, to limit the equipment to a rigid time schedule to duplicate calibration conditions would be unrealistic. Because of the seriousness of these problems several CT equipment makers have abandoned the use of these efficient solid state detectors in their latest designs.
If, on the other hand, thermal gradients within the detector array could be eliminated, all of the detectors would operate substantially at a common measurable temperature, and time would not be a factor. A single temperature measurement would accurately reflect the temperature of each detector, making individual temperature compensation for each measurement a simple matter. That is substantially what is accomplished by this invention.